This application aims to establish a causative role of skin structural proteins (keratin, collagen, and elastin) as endogenous UV-sensitizers in skin photodamage and evaluates modulation of these novel targets for chemoprevention of photocarcinogenesis and photoaging. Photooxidative stress initiated by solar irradiation of endogenous sensitizers is a key mechanism of skin photodamage, but the molecular nature of the photodynamic non-DNA chromophores responsible for light-driven production of reactive oxygen species (ROS) is unknown. Pilot data summarized in the application support the hypothesis that UV chromophores contained exclusively in extracellular matrix proteins of skin are potent endogenous photosensitizers. To test this hypothesis reconstructed epidermis, dermis, and full thickness human skin tissue equivalents will be used to allow quantitative control of protein UV-chromophores of both enzymatic and non-enzymatic origin (aim 1). Sensitizer-dependent and independent ROS production in response to total and fractionated solar UV light will be determined and results will be validated by direct chemical analyses (aim 2). Established molecular and cellular biomarkers of photooxidative stress will be assessed as a function of sensitizer content and radiation dose. Photosensitized enhancement of known biomarkers of photocarcinogenesis and photoaging will be studied (aim 3). The therapeutic potential of physical quenchers of photoexcited states as a novel class of topical chemopreventive agents for suppression of sensitized photodamage in reconstructed human skin will be evaluated. Reconstructed human skin with controlled sensitizer content will be irradiated and the chemopreventive potential of prototype quenchers as direct molecular antagonists of photoexcited states will be evaluated. A novel secondary amine pharmacophore, which protects human skin cells against photosensitization by physical deactivation of excited state molecules and singlet oxygen without chemical depletion, will be tested as a prototype chemopreventive agent and derivatized for optimal topical delivery (aim 4). The efficacy of the lead quencher of photoexcited states will be tested in an animal model of photocarcinogenesis. This study will validate endogenous structural skin components as novel molecular targets for chemoprevention in skin photocarcinogenesis and photoaging and lead to a new class of photoprotective agents.